The present invention relates to a digital X-ray imaging system and more particularly to an X-ray computed tomography system for medical use and a multi-elements solid-state radiation detector used for an X-ray scanography system, a bone mineral densitometer, a baggage inspection system, and the like.
In recent days, the needs have being risen for improving the quality of an image displayed in a digital X-ray imaging system, in particular, an X-ray computed tomography system used for an X-ray computed tomography for medical use, an X-ray scanography system, a bone mineral densitometer or a baggage inspection system. To improve the quality of an image, it is indispensable to enhance the performance of an X-ray detector served as a key component of such, a device. Today, the main stream of the X-ray detector is an xenon ionization detector, the performance of which has been improved up to a limit in light of the operating principle and structure. In practice, therefore, it is quite difficult to enhance the density and the sensitivity of the xenon ionization detector. As such, in place of the xenon ionization detector, today, a high-performance multi-elements solid-state radiation detector has been proposed. This detector is arranged to have a plurality of X-ray detection elements, each of which is composed of a scintillator and a photodiode served as a photoelectric conversion element. Remarks have been focused on this detector in that it can provide a more excellent S/N of an image than the ionization detector. FIG. 2 shows one example of a fundamental structure of a multi-elements solid-state radiation detector to which the present invention relates. The radiation detector is comprised of a scintillator 2 for converting X-ray 6 incident to the detector into light, a photodiode 3 for converting the light into an electric signal, an isolation plate 1 for isolating the adjacent X-ray detection elements from each other, a reflective plate at front window 5 for transmitting incident X-ray, cutting off light incoming from the outside of the detector, reflecting the light applied from the scintillator 2 and guiding the reflected light into a photodiode 3, and a print circuit board 4. The performance of the detector is evaluated by its quantum efficiency. To enhance the quantum efficiency, it is important to enhance the light collection efficiency of the scintillator 2, the photoelectric conversion efficiency of the photodiode 3, the X-ray spatial efficiency of the detector, and the light transmission efficiency inside of the detector.
To enhance the X-ray spatial efficiency, it is necessary to reduce a ratio of dead space to overall space on the detector, that is, an area except the scintillator 2 in which no contribution is made to detecting an incident X-ray, concretely, the isolation plate 1 for isolating the adjacent elements from each other as shown in FIG. 2. To enhance the light transmission efficiency, it is important to reduce absorption of light inside of the scintillator 2 and on the surface of the isolation plate 1 and the reflective plate at front window for the purpose of efficiently guiding light to the photodiode 3.
The spatial resolution on a reconstructed image formed by the X-ray computed tomography depends on a width of an X-ray detection element of the detector (a spacing between the isolation plates in FIG. 2). Today, many of X-ray computed tomography systems have a width which is as narrow as 1 mm or less for improving the spatial resolution. A narrower width of an X-ray detection element results in making radiation dose incident to each detection element smaller, thereby reducing an output signal of the detection element according to the radiation dose, and lowering its S/N. To keep lowering of the S/N to a minimum, various results are required to be achieved.
As the width of the X-ray detection element is made narrower, the quantity of light applied from the scintillator 2 directly to the photodiode 3 is made smaller. As the light is travelling to the photodiode 3 through a light path, the light is reflected on the surface of the isolation plate 1 and the surface of the reflective plate at front window 5 in various directions and is refracted lot of times within the scintillator 2. As such, it is impossible to keep a reflectance on the surface of the reflective plate at front window 5 and the isolation plate 1 to 100%. Each reflection entails light absorption, so that the detection efficiency may be inevitably lowered. In a case that, therefore, the reflectance on the surface is made lower, the absorptive quantity of light is increased so that the detection efficiency is made lower accordingly. It is therefore necessary to enhance the reflectance on the surface of the reflective plate at the front window and the isolation plate.
Consider that the surfaces of the reflective plate at the front window 5 and the isolation plate 1 have fine concaves and convexes and serve as diffusible reflectors. On the diffusible reflector, the reflected light is scattered so that it may be expanded in the light path. This results in increasing the number of reflections on the surface, thereby lowering the detection efficiency. Hence, it is preferable that the isolation plate 1 has so smooth a surface that it may be specular-reflective. However, it is quite difficult to obtain direct reflection on the surface of the isolation plate 1. It means that the performance of the isolation plate 1, that is, the performance of the detector depends on how much its surface comes closer to specular reflection.
As prior art for enhancing an X-ray spatial efficiency and a light transmission efficiency, proposals have been made in JP-A-1-202684 and JP-A-58-219471. In the former proposal, for obtaining the isolation plate 1, a metallic thin film made of Ta, W or Wo is chemical etched to a predetermined form. A different metal from that of the isolation plate, such as Cu, Ni or Cr is plated on the etched metal film and is made mechanically smoother. On the resulting film, there is formed a light reflective film for improving a reflectance of the surface. In the latter proposal, a reflective isolation plate is formed by filling a mixture of a light reflective agent (barium sulfate, titanium dioxide, etc.) and an adhesive agent between the adjacent X-ray detection elements.